IN August, Fidel Castro will turn 80, with no final reward in sight. The small island nation he has tyrannized for an astonishing 47 years has played an outsize role in modern history, from the 1962 Cuban missile crisis, which brought the superpowers "thisclose" to nuclear war (in the words of Robert McNamara), to the Elián González case, which helped tip Florida and thus the 2000 presidential election to George W. Bush.

At a deeper level, Castro has influenced the American culture wars of the last half-century. The beard and fatigues he presented to the world in 1957 anticipated the rebellious romanticism of the 1960's. The curdling of the Cuban revolution offered at least some vindication to the American right, while extending the ferocity of American ideological combat long past the end of the cold war. And, as we learn in Anthony DePalma's fascinating and admirably dispassionate book "The Man Who Invented Fidel," today's tussles over the "liberal media" in general, and The New York Times in particular, are merely an extension of an old story from the precomputer age -- a story that helped create Castro and, even now, illuminates the enduring power of bias and myth.

The explosive consequences of reporters growing too fond of their sources are all on display in the case of Herbert L. Matthews, one of the most famous -- and infamous -- men ever to write for this newspaper. Born in New York in 1900, Matthews barely missed combat in World War I and thought about becoming an academic. Instead, he drifted into secretarial work at The Times and by the 1930's was a foreign correspondent -- "brave as a badger," in the words of his friend Ernest Hemingway. Matthews's journalistic hero was Richard Harding Davis, the swashbuckling turn-of-the-century correspondent known as Theodore Roosevelt's "personal publicist" for creating the myth that T.R.'s charge up San Juan Hill was a pivotal battle in the Spanish-American War.

Matthews's first big story for The Times was the 1935 Italian invasion of Abyssinia, where he openly sympathized with Mussolini's Fascists. In Spain the next year, he switched sides and drew close to the Loyalist cause. Hemingway's wife, the journalist Martha Gellhorn, believed Matthews was the model for Robert Jordan in "For Whom the Bell Tolls." To the end of his life, members of the Abraham Lincoln Brigade, made up of American leftists and Communists who took part in the Spanish Civil War, considered him a sympathetic friend.

In 1957, Matthews was an aging Times editorial writer whose strong relationship with the publisher, Arthur Hays Sulzberger, allowed him to double as a roving straight-news reporter, an arrangement that was a departure from Times policy. After The Times and other newspapers reported that the young Cuban rebel leader Fidel Castro was dead, Matthews -- always a resourceful, enterprising correspondent -- decided to go see for himself. Posing as tourists, he and his wife made their way through the dictator Fulgencio Batista's military lines before Matthews alone completed the difficult journey into the Sierra Maestra on foot.

The front-page scoop that followed and two additional articles predicted "a new deal for Cuba" if Castro's insurgency won and reported that the romantic revolutionary was no Communist; in fact, the local Communists opposed him. The exclusive was a sensation at the time and transformed Castro's image from a hotheaded Don Quixote into the youthful face of the future of Cuba. Unfortunately for Matthews and The Times, it didn't age well.

By 1958, Times editors were already growing uncomfortable with Matthews's pro-Castro bias, and by 1959, when Castro credited the articles with helping to bring him to power, the remarkable access afforded Matthews began to boomerang. On a celebrated visit that year to the United States, the charming new Cuban leader bragged that when Matthews met him in the mountains two years earlier his movement was down to 18 soldiers -- one bedraggled column that walked in circles to fool the reporter. DePalma shows that this was almost certainly untrue -- one of Fidel's cruel jokes -- and that Matthews's larger estimates of Castro's troop strength came from careful reporting in Havana. But the damage to Matthews's reputation was done. For all the years since, conservatives who distrust everything coming out of Castro's mouth have chosen to believe their enemy on this single point, so as to make a fool of Herbert Matthews.

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His career did not crater all at once. In 1961, John F. Kennedy asked him to the Oval Office after the failure of the C.I.A.-backed invasion at the Bay of Pigs. A candid president, trying to learn from his mistakes, had earlier told The Times's managing editor, Turner Catledge, that "you would have saved us from a colossal mistake" if the paper had gone ahead and printed what it knew about the operation beforehand -- a sharp contrast to President Bush's attitude toward critical reporting. In his private chat with Matthews, unearthed by DePalma, Kennedy told the reporter that if it hadn't been for the failed invasion, "we might be in Laos now -- or perhaps unleashing Chiang." In other words, the botched invasion of Cuba may have spared the United States a much more disastrous invasion of mainland China.

DePalma shows that Matthews was a determined liberal but not a faker like Walter Duranty, the Times correspondent who won a 1932 Pulitzer Prize for his fawning coverage of Stalin and was probably in league with the Soviet secret police. Matthews's articles were for the most part factually accurate. But he comes across as a self-righteous and credulous analyst who sided with those who gave him access and then refused to reassess, whatever the changing facts. While other reporters who also misread Castro toughened their coverage after he began ordering summary executions, Matthews stuck stubbornly to his original myth.

While his pacing and historical context are first-rate, DePalma -- himself a correspondent at The Times -- might have quoted more extensively from the many rationalizations of Castro that Matthews undertook in books and articles in the 20 years between the famous interview and his death in 1977. Did he ever go beyond comparisons to Oliver Cromwell and John Brown and call Castro by his proper name -- dictator? Apparently not, though DePalma doesn't say.

The Matthews story is about the power of myths. The most enduring on the left are that the United States drove Castro into the hands of the Soviets (DePalma explores documents from the Soviet archives that suggest the Soviets offered to send military trainers into Cuba well before the relationship with the United States deteriorated) and, most perniciously, that there is still something romantic and appealing about the Cuban revolution. The most persistent myths on the right are that a trade embargo makes sense (it actually helps perpetuate Castro's power) and that the dictator is just a garden-variety Communist; in fact, he has always been an original and unpredictable chameleon whose only commitment is to his own survival.

Herbert L. Matthews didn't invent Castro, as he initially claimed with characteristic self-regard. As DePalma suggests, Castro's charm and will to power were such that he most likely would have triumphed without Matthews's notorious articles turning him into a romantic hero. The rabid Cuban exiles who continue to revile the reporter nearly 30 years after his death simply wanted to shoot the messenger. (Some literally: the F.B.I., while spying on Matthews, also reported a death threat against him.) But Matthews's critics were more right than wrong, and his career is an object lesson to anyone with aspirations to being an accomplished reporter. Passion for sources and causes can make you famous, but they often pull you farther from the brambled path of truth.

'The Man Who Invented Fidel,' by Anthony DePalma Jonathan Alter, a senior editor and columnist at Newsweek, is the author of "The Defining Moment: FDR's Hundred Days and the Triumph of Hope," to be published next week.